1. Field of the Invention
This invention relates to new and improved catalysts for the production of vinyl acetate (VA) by reaction of ethylene, oxygen and acetic acid.
2. Background Information Including Description of Related Art
It is known to produce vinyl acetate by reaction of ethylene, oxygen and acetic acid using a catalyst consisting of metallic palladium, gold and copper supported on a carrier (for example, see U.S. Pat. No. 5,347,046 and U.S. Pat. No. 5,731,457). While the process utilizing such a catalyst is capable of producing vinyl acetate at fair levels of productivity, any expedient capable of achieving even greater productivity over the life of the catalyst is obviously advantageous.
More particularly the foregoing catalysts comprising metallic palladium, gold and copper may be prepared by a process including the steps of impregnating a porous support with a single aqueous solution or separate solutions of water-soluble salts of these metals, reacting the impregnated water-soluble salts with an appropriate alkaline compound e.g., sodium hydroxide, to "fix" the metals as water-insoluble compounds, e.g. the hydroxides, and reducing the water insoluble compounds, e.g., with ethylene or hydrazine, to convert the metals to free metallic form. This type of process has the disadvantage of requiring several steps, sometimes including at least two "fixing" steps.
The following references may be considered material to the invention claimed herein. U.S. Pat. No. 5,332,710, issued Jul. 26, 1994, to Nicolau et al., discloses a method of preparing a catalyst useful for the production of vinyl acetate by reaction of ethylene, oxygen and acetic acid, comprising impregnating a porous support with water soluble salts of palladium and gold, fixing the palladium and gold as insoluble compounds on the support by immersing and tumbling the impregnated support in a reactive solution to precipitate such compounds, and subsequently reducing the compounds to free metallic form.
U.S. Pat. No. 5,347,046, issued Sep. 13, 1994 to White et al., discloses catalysts for the production of vinyl acetate by reaction of ethylene, oxygen, and acetic acid, comprising a palladium group metal and/or a compound thereof, gold and/or a compound thereof, and copper, nickel, cobalt, iron, manganese, lead or silver, or a compound thereof, preferably deposited on a support material.
British Patent No. 1 ,188,777, published Apr. 22, 1970 discloses a process for the simultaneous production of an unsaturated carboxylic acid ester, e.g. vinyl acetate, from an olefin, carboxylic acid, and oxygen, and the corresponding carboxylic acid, e.g., acetic acid, from its aldehyde, using a single supported catalyst containing a palladium compound, e.g. an oxide or salt, with one or more compounds of any of various metals, e.g. metallic gold or a gold compound such as potassium aurate.
U.S. Pat. No. 5,700,753 discloses vinyl acetate (VA) catalyst prepared by adding organometallic gold complexes to prereduced palladium catalyst prepared from Na.sub.2 PdCl.sub.4. The organometallic gold compound does not require a fixing procedure.
U.S. Pat. No. 5,731,457 describes a VA catalyst prepared with non-halogen containing copper compound.